Messages in this thread | | | Date | Fri, 29 Apr 2022 15:37:30 +0200 | Subject | Re: [PATCH] rpmsg: mtk_rpmsg: Fix circular locking dependency | From | AngeloGioacchino Del Regno <> |
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Il 28/04/22 19:31, Mathieu Poirier ha scritto: > On Fri, Jan 14, 2022 at 03:47:37PM +0100, AngeloGioacchino Del Regno wrote: >> During execution of the worker that's used to register rpmsg devices >> we are safely locking the channels mutex but, when creating a new >> endpoint for such devices, we are registering a IPI on the SCP, which >> then makes the SCP to trigger an interrupt, lock its own mutex and in >> turn register more subdevices. >> This creates a circular locking dependency situation, as the mtk_rpmsg >> channels_lock will then depend on the SCP IPI lock. >> >> [ 18.014514] Possible unsafe locking scenario: >> [ 18.014515] CPU0 CPU1 >> [ 18.014517] ---- ---- >> [ 18.045467] lock(&mtk_subdev->channels_lock); >> [ 18.045474] lock(&scp->ipi_desc[i].lock); >> [ 18.228399] lock(&mtk_subdev->channels_lock); >> [ 18.228405] lock(&scp->ipi_desc[i].lock); >> [ 18.264405] > > I finally understand the problem, something that would have been impossible > without the pastebin you provided in your latest email. Please add the content > of that pastebin to the changelog and send another revision (checkpatch > warnings can be ignored). >
Thanks for giving it another look... I'll add a cover letter with the contents of the pastebin to avoid possible confusion for anyone looking at the patch.
>> >> To solve this, simply unlock the channels_lock mutex before calling >> mtk_rpmsg_register_device() and relock it right after, as safety is >> still ensured by the locking mechanism that happens right after >> through SCP. > > The integrity of the subdev->channels list is guaranteed by relocking the > mutex, I'm not sure what "through SCP" adds to the sentence.
I'll clarify the commit message.
> >> Notably, mtk_rpmsg_register_device() does not even require locking. >> > > I don't agree with the above sentence - if locking doesn't happen in > mtk_rpmsg_create_device(), there can be two CPUs accessing the list at the same > time. >
That's right, I have largely underestimated that for some reason, sorry about that.
Regards, Angelo
> Thanks, > Mathieu >
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